Recently, interests in energy storage technology are gradually increased. As the use of batteries is enlarged to applications for the storage of energy for portable telephones, camcorders, notebook computers, personal computers and electric vehicles, efforts on the research and development of the batteries are increasingly embodied. In this view, the field of electrochemical devices receives the greatest attention, and among them, interests in the development of chargeable/dischargeable secondary batteries are focused.
Among secondary batteries which are now in use, lithium secondary batteries developed in the early 1990s are in the spotlight due to the advantages of higher operation voltages and far greater energy densities than those of conventional batteries, such as Ni—MH, Ni—Cd and sulfuric acid-lead batteries. However, the lithium secondary batteries are disadvantageous in that they have safety problems, such as firing and explosion, caused by the use of organic electrolytes, and their preparation requires a complicated process.
Meanwhile, ionic liquid, also called “room temperature molten salt”, means a salt showing liquid properties at room temperature. The ionic liquid consists generally of organic cations and inorganic anions and is characterized by having high evaporation temperature, high ion conductivity, thermal resistance, nonflammability and the like. The ionic liquid is applied in solvents for organic synthesis, solvents for separation and extraction, and the like, and recently, there are studies on the possibility for the application of the ionic liquid to an electrolyte solution for electrochemical devices, such as capacitors, lithium ion batteries, fuel batteries and the like. Most of such studies concern electrolyte solutions for capacitors, and many studies on the application of the ionic liquid to an electrolyte solution for lithium ion batteries are now conducted in Japan and USA as leaders, but not yet put to practical use due to the problems of a reaction between the ionic liquid and the carbon-based anode, and an increase in the viscosity of the electrolyte solution.
U.S. Patent Application No. 2002-0110737 discloses the application of the ionic liquid to an electrolyte solution for lithium ion batteries. This patent application describes that, by the selection of a suitable ionic liquid and the control of the ratio of the selected ionic liquid to the existing electrolyte solution, the ionic liquid can show nonflammability. Also, it describes that the reaction between the ionic liquid and the carbon-based anode can be inhibited by a process for injecting the electrolyte solution in two steps. However, the two-step process for injecting the electrolyte solution has problems in that the process is difficult to be actually applied and to produce an ionic liquid whose reaction with an anode (negative electrode) has been inhibited. Also, due to the high viscosity of the ionic liquid, an increase in the viscosity of the mixed electrolyte solution cannot be avoided, thus causing the deterioration in the battery performance.
Furthermore, Japanese Patent Laid-Open Publication Nos. Hei 11-86905 and 11-260400 disclose the application of an imidazolium cation-containing liquid to an electrolyte solution for ion batteries. However, the ionic liquid used in such patents has a problem in that it shows a higher reduction potential than that of lithium ions so that it is reduced faster than lithium ions in the anode.
In an attempt to solve the problem of the high reduction potential of the ionic liquid as described above, Japanese Patent Laid-Open Publication No. Hei 11-297335 discloses an ionic liquid based on ammonium with a lower reduction potential than that of lithium. In this case, the reduction potential problem can be overcome, but there is a problem in that the ionic liquid is co-intercalated with lithium ions into the carbon-based anode.
In an attempt to solve the problem of the co-intercalation of the ionic liquid with the carbon-based anode, Japanese Patent Laid-Open Publication No. 2002-110225 discloses the use of a titanium-based anode. However, even in this case, there is a problem in that, due to the high viscosity of the ionic liquid, the high-efficiency discharge performance of batteries will be deteriorated when the ionic liquid is applied in the batteries.